Mothers are more likely to suffer from serious morning sickness if they are pregnant with a girl, researchers claim.

A study of over a million pregnancies in Sweden between 1987 and 1995 showed there was a greater risk of the condition for women who were carrying a female child.

Though there were more births of boys during the period studied, there was a higher chance of mothers-to-be of girls being admitted to hospital suffering from hyperemesis gravidarum - serious nausea and vomiting during the first three months of pregnancy.

The female to male ratio of hospital admissions was 56:44, according to the research carried out by Dr Johan Askling at Stockholm's Karolinska Institute.

Girl babies may produce different hormones, that could be part of the problem

Dr Michael De Swiet

The cause of morning sickness is not known, but it is thought to be related to increased production of a pregnancy-related hormone called human chorionic gonadotropin.

Despite the results of the survey, Dr Askling said parents should not use serious sickness as a sign that they were to have a female child. "Using sickness to predict the sex of an individual offspring is not much better than tossing a coin," he told The Lancet medical journal.

Results significant

Dr Michael De Swiet at the Institute of Obstetrics and Gynaecology at Queen Charlotte's and Chelsea Hospital, London, said the size of the study meant its results were significant.

"There could be all sorts of explanations," he said. "Morning sickness is something to do with hormones - girl babies may produce different hormones, that could be part of the problem.

"There are a number of illnesses in pregnancy that are triggered by the baby and there are differences between boys and girls."

Acute fatty liver of pregnancy, for instance, is more common with boy babies.

He questioned whether the results of the study would have any particular use for parents or doctors in the short-term, though it could point scientists towards a cause for the condition in the future.